This invention relates to sensors or detectors for radiation, including light, X-rays and heat, and for pressure and electric fields.
Sensors for various kinds of radiations are known in the art, but they are deficient in a number of different ways. For example, the photoresponse of known semiconductor sensors in the ultraviolet (UV) and X-ray radiation range is limited due to surface recombination.
The radiation with energy more than a few hundred meV above the band gap will be absorbed in a thin layer (.about.100 .ANG. deep) near the surface. The photogenerated free carriers due to I/V/X-ray absorption recombine non-radiatively in the vicinity of the surface of the semiconductor. This is in contrast to visible/infrared (IR) radiation where the absorption length of the radiation is significantly larger and the recombination occurs deeper in the bulk away from the surface. The carrier recombination at the surface may be avoided by one of the following schemes: (i) by passivating the surface of the semiconductor using a chemical treatment. For example, in porous Si the surface recombination velocities could be reduced to as low as 0.1 cm/sec in HF-treated surface and significant light emission can be observed; or (ii) the surface of the semiconductor absorber in question can be passivated using hetero-junction schemes. This is evidenced in many III-V semiconductor devices.
The above two schemes are not available when the impinging radiation is of very high energy (even compared to passivating- layer band gap) such as of UV, X-ray and synchrotron radiation. In all of the above, the detection of the photo-generated carriers is observed via a scintillation process using an impurity or dopant ion as a luminescent center. This energy transfer of carriers between the host to the luminescent center is not very efficient. The reasons for the low efficiency, in the most simple yet instructive description, is that the radiation-generated electron and hole in the host material are not localized. Their wavefunction is spread out over a range comparable to the excitonic Bohr radius of the material, a.sub.B (.about.100 .ANG. in semiconductors). On the other hand, the optically-active electron of the dopant ion belonging to the shielded f (rare earth) or d (transition metals) shell, is localized in a radius of only a few angstroms. Thus, the overlap of the wave functions between the host and donor is small, and thus the probability of transition between them is much smaller than the probability of recombination. A more detailed study reveals that in the vicinity of the dopant atom, the wave function of the electron in the conduction band has an s-orbital shape and the wave function of the hole in the valence band has a p-orbital shape. Since the impurity electron has a d-orbital or f-orbital shape, some of the transitions are prohibited by symmetry conservation.
Present time chest x-rays are not available for the early diagnosis of, for example, breast cancer because of its relatively high, 0.1 rad, dosage. In principle, a dosage reduction while preserving the quality of the image would offer the benefit of using early x-ray diagnostics with appropriate frequency. In addition, it is imperative to have a high resolution technique to detect a lump on a much smaller scale. Also, radiographic imaging remains unattractive because the films consumed are highly uneconomical. On the other hand, one can digitize the X-ray image to the resolution needed for a CRT display, yet the signal to noise ratio in the CRT itself limits the final resolution. Besides developing a high brightness and high resolution CRT, it is necessary to improve the detection methods and sensitivity currently available in X-ray imaging for lower X-ray dosage. To improve detection sensitivity, a large effort is devoted to collimating the X-rays by different techniques which allow to reduce the 1/R.sup.2 loss of the intensity to a minimum. For example, development in the area of Kulakov lens and a free electron laser source for improved X-ray collimated source are being considered for the next generation of high resolution and low dosage systems. Simultaneously radiation-hardened detectors based on charge-injection devices are being developed. These can withstand a higher X-ray exposure for a prolonged time. However the size of the detector limits the overall resolution. For mammography applications, a detector size of at least 10 cm.times.10 cm is required.